Simulation of natural aging property of microplastics in Yangtze River water samples via a rooftop exposure protocol

Due to low aging rate, the environmental behavior of naturally weathered microplastics (MPs) are not fully understood. Here, we systematically investigated the surface property and adsorption behavior of polypropylene (PP), polyethylene (PE) and polystyrene (PS) MPs during outdoor exposure in Yangtze River water for 18 months, and compared their difference from those in laboratory process. Results showed that compared to PE and PS MPs, PP underwent higher changes in surface aging properties such as rapid fragmentation and surface oxidation. Outdoor exposure exhibited different effects on adsorption property of MPs for metal ions, where adsorption capacities of PE and PS MPs for Co(II) were increased with aging degrees, while few change occurred on different aged PP MPs. As for Cr(VI), aging process further decreased the overall adsorption on PP, PE and PS MPs. The difference was mainly ascribed to the surface property (e.g. oxygen-containing groups and size) and the adhered biofilm and charged minerals. We further validated that similar types of oxidation products were formed between natural and laboratory aging of MPs, whereas the reaction order of these products, fragmentation rate and the change in adsorption property of aged MPs might be different in both processes. The findings provide essential information to assess real environmental behavior of MP samples.